Aneuploidy is the leading cause of miscarriages and mental retardation in humans. Determining the causes of meiotic chromosome mis-segregation is thus vital for understanding the principles underlying Down's Syndrome and infertility. The long- term goal of our studies is to define the mechanisms that prevent chromosome mis-segregation and thus aneuploidy during meiosis. Meiosis is a specialized cell division during which a single DNA replication phase is followed by two consecutive chromosome segregation phases (meiosis I and meiosis II). Meiosis-specific factors act on the cyclin-dependent kinase (CDK) Cdc28 associated with the six Clb cyclins (Clb-CDKs) to bring about the unique meiotic cell division program. This grant has two goals. (1) We want to characterize how Clb-CDKs control meiosis. (2) We want to characterize the changes that occur on CDKs and its regulators during meiosis and to identify the meiosis-specific regulators that cause these changes. In Specific Aim 1 we will determine the mechanisms that prevent Clb1-CDKs from being active during meiosis II and examine the importance of this regulation. In Specific Aim 2 we will investigate how translation of the cyclin CLB3 is controlled. Inhibition of CLB3 translation is essential for meiosis I chromosome segregation. Thus, understanding how CLB3 translation is controlled and how CLB3 inhibits meiosis I is essential to understand how a meiosis I chromosome segregation pattern is established. In Specific Aim 3 we will determine how CLB3 inhibits meiosis I chromosome segregation. Since the basic cell division machinery is highly conserved from yeast to human and given that most of the factors described in this proposal have also been shown to control meiosis in mammals, it is likely that the processes governing meiosis will also be conserved across species. Thus, as with many other cell cycle studies, the regulatory processes discovered and characterized in yeast will likely guide the way for studies in higher eukaryotes including human. Public Health Relevance: Aneuploidy generated during meiosis is the leading cause of mental retardation and miscarriages in humans. Determining the molecular mechanisms governing meiotic chromosome segregation and the surveillance pathways that ensure that gamete formation occurs with accuracy is thus vital for understanding the principles underlying chromosome mis-segregation leading to Down's Syndrome and infertility. The long-term goal of our studies is to define the mechanisms that control meiotic chromosome segregation and gamete formation at the molecular level.